<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #father {
            width: 500px;
            height: 300px;
            background-color: skyblue;
            position: relative;
        }

        #son {
            width: 100px;
            height: 100px;
            background-color: green;
            position: absolute;
            top: 0;
            right: 0;
            bottom: 0;
            left: 0;
            margin: auto;
        }
    </style>
</head>

<body>
    <div id="father">
        <div id="son">
        </div>
    </div>
</body>

</html>